1. Field of the Invention
The present invention generally relates to an apparatus for defrosting frozen foods and, more particularly, to improvements in the defrosting apparatus which is capable of defrosting substantially simultaneously a relatively large number of frozen foods of relatively small size as uniformly as possible with a short time.
2. Description of the Prior Art
Heretofore, in conjunction with the defrosting (or defreezing) of frozen foods, there have been proposed numerous types of defrosting apparatuses having various structures and configurations. FIGS. 6 and 7 of the attached drawings show schematically a typical structure of a hitherto known defrosting apparatus. In these figures, a reference numeral 1 generally denotes a box-like housing of a thermally insulated structure. Mounted on the top surface of this housing 1 are a compressor 3, a condenser 4 and a fan motor 5 which constitute major parts of a refrigerating unit 2. Also, a vaporizer or evaporator 6 also constituting a part of the refrigerating unit 2 is disposed at an upper position within the housing 1. A heating device 7 constituted by an electrical heater or the like is disposed beneath the evaporator 6 with a ventilating fan or blower 8 being mounted to the left of the evaporator 6 and the heater 7.
In the interior of the housing 1, a storage space for storing or accommodating therein frozen foods is defined by a horizontal shield plate 9 disposed below the heating device 7 and a pair of shelf receiving members 10 disposed vertically in opposition to each other and in laterally spaced relationship with the inner side walls of the housing 1. Each of the shelf receiving members 10 is provided with a plurality vertically spaced, L-shaped supporting fixtures 13, and shelves 14 are disposed between the paired shelf receiving members 10 while being supported on the respective supporting fixtures 13. The frozen foods 15 are placed on the shelves 14. A transverse air inlet port 11 is provided between one end of the shield late 9 and one end of the topmost shelf plate 14, while a transverse air outlet port 12 is provided between the other end of the shield plate 9 and the topmost shelf plate 14. Furthermore, openings corresponding to the inlet ports 11 and the outlet ports 12 are also formed in the shelf receiving members 10 between the adjacent shelves 14, these openings being denoted by the same reference numerals as the ports mentioned above, as will be seen in FIG. 7 (in which only the inlet ports or openings 11 are visible). The frozen foods 15 on the shelf plates 14 can be taken out from the housing 1 by opening a door(s) 16 mounted pivotally on the housing.
In operation, when the heating device 7 is activated simultaneously with the ventilating fan 8, the air within the housing 1 is first heated by the heating device 7, and the hot air resulting from the heating is forced to flow along paths indicated by arrows A, C, E and G through operation of the ventilating fan 8. More specifically, the hot air flows into the storage space through the air inlet ports or openings 11 mentioned above, and heat exchange takes place between the hot air and the frozen foods 15 on shelves 14 within the storage space. Thereafter, the air is expelled from the storage space through the outlet ports 12 as indicated by arrows B, D, F and H to be fed back toward the ventilating fan 8 for subsequent recirculation within the housing 1 in the manner described above.
In the prior art defrosting apparatus of the type described above, results of actually performed measurement of the hot-air temperature at the positions corresponding to the individual air inlet ports or openings 11 show that the temperature of the hot air becomes higher the higher the air inlet port is positioned, because high temperature air naturally exhibits a tendency to rise higher than low temperature air. Consequently, a longer time is required for defrosting the frozen foods placed on the lower shelves when compared with those positioned on the higher shelves, resulting in remarkable non-uniformity in the degree of defrosting among the foods to be defrozen. Under these circumstances, it was either necessary to put up with such non-uniformity in the defrosted state among the foods or alternatively to extend duration of the defrosting operation until the frozen foods placed on the lowermost shelf have been defrosted.
The defrosting apparatus can be operated as a cooling or refrigerating apparatus by causing a liquid coolant to flow through the evaporator 6 after the completion of the defrosting operation. In this case, when the defrosting period is extended for the reason mentioned above, the foods placed on the shelf of high level or position which are naturally defrosted earlier than those placed on the lower shelves would be rid of moisture, allowing surface fading or discoloration just about the time the foods on the lower shelf have been completely defrosted. It goes without saying that such faded or discolored foods are significantly reduced in commercial values.
Further, in the case where a relatively small amount of food is stored in the housing 1, there may be regions on the shelfs located along the flow path of the circulating hot air in which there are no foods to be defrosted. In that case, the hot air is fed back to the ventilating fan 8 through the associated outlet port 12 without undergoing heat exchange, which brings about the following. Namely, some defrosting apparatuses of the type described above are equipped with a temperature sensor on the air-intake side of the ventilating fan 8 for the purpose of determining the point in time at which the defrosting operation is to be terminated. When a predetermined temperature is detected by the sensor, a defrosting operation terminating signal is generated. Accordingly, when the situation mentioned above takes place in this type of defrosting apparatus, the sensor may detect the temperature of the hot air which has not undergone heat exchange with any foods, whereby a defrost inhibit signal would be produced to trigger the succeeding cooling or refrigerating operation notwithstanding the the fact that the frozen foods have not yet been satisfactorily defrosted. This means that a great difficulty is encountered in performing the automatic defrosting operation for a small amount of frozen foods.